// drawPencil takes an (x, y) position (from a MotionNotify event) and draws
// a rectangle of size pencilTip on to canvas.
func drawPencil(canvas *xgraphics.Image, win *xwindow.Window, x, y int) {
// Create a subimage at (x, y) with pencilTip width and height from canvas.
// Creating subimages is very cheap---no pixels are copied.
// Moreover, when subimages are drawn to the screen, only the pixels in
// the sub-image are sent to X.
tipRect := midRect(x, y, pencilTip, pencilTip, width, height)
// If the rectangle contains no pixels, don't draw anything.
if tipRect.Empty() {
return
}
// Output a little message.
log.Printf("Drawing pencil point at (%d, %d)", x, y)
// Create the subimage of the canvas to draw to.
tip := canvas.SubImage(tipRect)
// Now color each pixel in tip with the pencil color.
tip.For(func(x, y int) xgraphics.BGRA {
return xgraphics.BlendBGRA(canvas.At(x, y).(xgraphics.BGRA), pencil)
})
// Now draw the changes to the pixmap.
tip.XDraw()
// And paint them to the window.
tip.XPaint(win.Id)
}
